1. Field of the Invention
The present invention relates to an optical scanning apparatus to be mounted on an image forming apparatus such as a copier or a printer, and in particular, it relates to an optical scanning apparatus mounted with a so-called double path optical system, in which laser beam (light beam) generated from a light source pass though a scanning lens, and after that, are deflected by a rotation polygon mirror, and then, pass through the scanning lens again, and emit toward a surface to be scanned (photosensitive member).
2. Related Background Art
To increase the number of output sheets of the printer and the copier per unit hour, a scanning speed on the photosensitive member by optical beams must be increased. As a way to increase the scanning speed, though there exist a method of increasing the rotational speed of the rotation polygon mirror, a method of providing a plurality of light sources and turning them into multi beams, and the like, an optical scanning apparatus of an over field type which can gain a number of reflecting surfaces while controlling the diameter of the rotation polygon mirror is also known as one of techniques of increasing the scanning speed. The optical scanning apparatus of the over field type is characterized in that the width in the main scanning direction of the laser beam which comes into the rotation polygon mirror is larger than the width in the main scanning direction of one of the surfaces of the rotation polygon mirror.
In the optical scanning apparatus of this over field type, since a F number of the optical beams deflected by the rotation polygon mirror varies according to the deflection angle, there occurs a phenomenon in which the beam diameter and the light amount of the optical beams become non-uniform along the main scanning direction on the surface to be scanned of the photosensitive member or the like. In order to mitigate this phenomenon, in the optical scanning apparatus of the over field type, it is common to use a system in which optical beams come into the rotation polygon mirror in front with respect to the main scanning direction and with an angle with respect to the sub scanning direction. At this time, to get rid of the limitation of an angle with which the optical beams come into it in the sub scanning direction of the optical beams to the rotation polygon mirror, the optical scanning apparatus of a so-called double path type has been also proposed, in which the optical beams pass through the scanning lens (fθ lens), and come into the rotation polygon mirror, and the optical beams reflected by the rotation polygon mirror transmit the scanning lens again. Japanese Utility Model Publication No. H06-035212 and Japanese Patent Publication No. H06-027902 disclose the optical scanning apparatus of the double path type.
Such optical scanning apparatus of the double path type is an optical scanning apparatus of the mechanism in which the laser beams are allowed to come into the scanning lens, and the outgoing light from the scanning lens is deflected by the rotation polygon mirror, and after that, are allowed to pass though the scanning lens again. The laser beams having passed through the scanning lens twice, after that, are guided to the photosensitive member as an image bearing member of the image forming apparatus, and form an electrostatic latent image on the photosensitive member.
However, usually, when the laser beams pass through the scanning lens, it is not that the whole of the laser beams pass through completely, but that the portion thereof is reflected on the surface of the scanning lens. When these reflected laser beams are reflected by any portion of the apparatus so as to reach the photosensitive member, there is a possibility that an unexpected electrostatic latent image is formed on the photosensitive member. Particularly, as described above, when the laser beams are allowed to pass through the scanning lens twice, reflected lights reflected on the surface of the scanning lens comparatively many times, and the possibility of the reflected light reaching the photosensitive member is also increased.
In particular, since the same light path always exists, the laser reflected by the scanning lens is irradiated at the same position of the center of the photosensitive member before it is deflected by the rotation polygon mirror. Although the amount of this reflected light is few, it becomes a cause of forming an unnecessary electrostatic latent image. Then, there is a possibility of lowering resolution of the image formed on the photosensitive member.
To reduce the above described problems, there is a case where an expensive anti-reflection coating such as a multilayer film and the like is extended on surface of a scanning lens 2. However, since the coating is extremely expensive, the cost of the apparatus is increased.
Hence, in Japanese Utility Model Publication No. H06-035212 and Japanese Patent Publication No. H06-027902, there is disclosed that a light shield plate to shield this reflected light is provided so that the reflected light by the fθ lens does not reach the photosensitive member.
The optical scanning apparatus of the double path type, as shown in FIG. 3, is an optical scanning apparatus in which the laser beam obliquely comes into a reflecting surface 2F of the rotation polygon mirror 2 in the sub scanning direction (direction orthogonal to the deflecting direction by the rotation polygon mirror). If the incident angle (oblique incident angle α) in the sub scanning direction is made large, an angle made by the laser beam (incident light beam Ri1) traveling to the rotation polygon mirror and the reflected light by the scanning lens 1 becomes large, and it is relatively easy to dispose the light shield plate at a position not to shield the incident light beam Ri1 and the scanning light beam Rs1 but to shield the reflected light alone.
However, when the oblique incident angle α is made large, non-uniformity (pitch non-uniformity P) of the intervals between the scanning lines on the photosensitive member due to eccentricity of the rotation polygon mirror is unpreferably increased. Further, when the oblique incident angle α is made large, the thickness of the optical scanning apparatus is increased, and the entire apparatus become large-sized.
In contrast to this, when the oblique incident angle α is made small, the pitch non-uniformity P of the scanning lines can be controlled, and the thickness of the apparatus can be controlled.
However, when the angle of oblique incident α is made smaller, the area in which the light shield plate can be arranged becomes narrow, and it becomes difficult to arrange the light shield plate at a position where the laser beam necessary for the image formation is not shielded.
Further, in recent years, in the full color image forming apparatuses, the image forming apparatus of a so-called tandem system lining up a plurality of image forming portions has been in widespread use. To reduce a cost of the image forming apparatus of this tandem type, there exists a system (1 BOX type or 2 BOX type and the like) which emits laser beam to a plurality of photosensitive members by one laser scanner unit. In case the laser scanner unit of such 1 BOX type or 2 BOX type adapts the above described double path system, since the number of incident light beams and scanning light beams is increased for one rotation polygon mirror, the area where the light shield plate can be arranged becomes further narrow, and it becomes difficult to arrange the light shield plate at a position where the laser beam necessary for the image formation is not shielded.